The present invention relates to a hydraulic circuit for producing signal pressure for controlling the operation of a transmission and, more particularly, relates to a hydraulic circuit for obtaining signal pressure corresponding to the rotation of an engine. The present invention may be used for the control of a starter clutch of a transmission, for example.
Generally in a transmission for a vehicle, a clutch for controlling motive power transmission when a vehicle is started or stopped is provided between an input side member driven by a motor and an output side member coupled to a vehicle. When a vehicle is started or stopped, the engagement of the clutch is controlled. It is generally known that the control of engagement is executed by controlling oil pressure. But oil control for the operation of such a starter clutch is mostly executed by detecting the rotational frequency of a motor and driving an electric control valve by a quantity corresponding to the rotational frequency. The rotational frequency of the motor is detected by a sensor, and the like.
In such an oil control system using an electric control valve, to avoid the failure of oil control in case of an electric failure--such as a malfunction in the control system when a valve spool is stuck open--a backup unit for generating signal pressure corresponding to the rotational frequency of a motor is typically provided to execute oil control. For the backup unit, a type using a pitot tube is known and, referring to FIG. 8, the configuration of the unit will be described below.
A disc pitot flange 82 is provided with an oil chamber 82a the outer periphery of which is bent on the side of the inner periphery and fixed to a rotating member 81 driven by a motor. Oil 83 is supplied to the oil chamber 82a via an oil delivery pipe 85 branching from an oil passage 84 and the detection part 86a of a pitot tube 86 is inserted into the oil 83. The pitot tube 86 is connected to an oil chamber 87c of a signal pressure generating valve 87. When an input member 81 is rotated, the pitot flange 82 is also integrally rotated, however, at this time, the oil 83 in the oil chamber 82a is also turned along the wall of the oil chamber 82a because of its viscosity. At this time, as the pitot tube 86 detects the dynamic pressure of the turned oil 83 and supplies oil 83 equivalent to the rotational frequency of the pitot flange 82 to the oil chamber 87c of the signal pressure generating valve 87, a spool 87a is driven right against a spring 87b. Hereby, signal pressure (pitot pressure) corresponding to the rotational frequency of a motor is output to an oil passage 88.
In Japanese Patent Unexamined Publication No. Hei. 6-26565, a hydraulic control for controlling lubrication oil corresponding to the rotational speed of a pulley of a V belt-type continuous variable transmission is disclosed and an example in which oil pressure corresponding to rotational speed is produced using a pitot tube is disclosed as prior art.
However, in a signal pressure generator using such a pitot tube as described above, as space for installing the pilot flange 82 is required, the size in the axial direction is increased by the dimention of the pilot flange quantity and a transmission itself is made larger. According to the hydraulic control disclosed in the above patent application, a pitot flange is not required and a transmission can be miniaturized. However, it is also considered that the oil pressure of oil discharged from a regulator valve has an effect upon the regulation of line pressure by the regulator valve.